1. Field of the Invention
This invention resides in the field of steel alloys, particularly those of high strength, toughness, corrosion resistance, and cold formability, and also in the technology of the processing of steel alloys to form microstructures that provide the steel with particular physical and chemical properties.
2. Description of the Prior Art
Steel alloys of high strength and toughness and cold formability whose microstructures are composites of martensite and austenite phases are disclosed in the following United States patents, each of which is incorporated herein by reference in its entirety:
U.S. Pat. No. 4,170,497 (Gareth Thomas and Bangaru V. N. Rao), issued Oct. 9, 1979 on an application filed Aug. 24, 1977
U.S. Pat. No. 4,170,499 (Gareth Thomas and Bangaru V. N. Rao), issued Oct. 9, 1979 on an application filed Sep. 14, 1978 as a continuation-in-part of the above application filed on Aug. 24, 1977
U.S. Pat. No. 4,619,714 (Gareth Thomas, Jae-Hwan Ahn, and Nack-Joon Kim), issued Oct. 28, 1986 on an application filed Nov. 29, 1984, as a continuation-in-part of an application filed on Aug. 6, 1984
U.S. Pat. No. 4,671,827 (Gareth Thomas, Nack J. Kim, and Ramamoorthy Ramesh), issued Jun. 9, 1987 on an application filed on Oct. 11, 1985
U.S. Pat. No. 6,273,968 B1 (Gareth Thomas), issued Aug. 14, 2001 on an application filed on Mar. 28, 2000
The microstructure plays a key role in establishing the properties of a particular steel alloy, and thus strength and toughness of the alloy depend not only on the selection and amounts of the alloying elements, but also on the crystalline phases present and their arrangement. Alloys intended for use in certain environments require higher strength and toughness, and in general a combination of properties that are often in conflict, since certain alloying elements that contribute to one property may detract from another.
The alloys disclosed in the patents listed above are carbon steel alloys that have microstructures consisting of laths of martensite alternating with thin films of austenite. In some cases, the martensite is dispersed with fine grains of carbides produced by autotempering. The arrangement in which laths of one phase are separated by thin films of the other is referred to as a xe2x80x9cdislocated lathxe2x80x9d structure, and is formed by first heating the alloy into the austenite range, then cooling the alloy below the martensite start temperature Ms, which is the temperature at which the martensite phase first begins to form, into a temperature range in which austenite transforms into packets consisting of martensite laths separated by thin films of untransformed, stabilized austenite. This is accompanied by standard metallurgical processing, such as casting, heat treatment, rolling, and forging, to achieve the desired shape of the product and to refine the alternating lath and thin film arrangement. This microstructure is preferable to the alternative of a twinned martensite structure, since the lath structure has greater toughness. The patents also disclose that excess carbon in the lath regions precipitates during the cooling process to form cementite (iron carbide, Fe3C) by a phenomenon known as xe2x80x9cautotempering.xe2x80x9d The ""968 patent discloses that autotempering can be avoided by limiting the choice of the alloying elements such that the martensite start temperature Ms is 350xc2x0 C. or greater. In certain alloys the carbides produced by autotempering add to the toughness of the steel while in others the carbides limit the toughness.
The dislocated lath structure produces a high-strength steel that is both tough and ductile, qualities that are needed for resistance to crack propagation and for sufficient formability to permit the successful fabrication of engineering components from the steel. Controlling the martensite phase to achieve a dislocated lath structure rather than a twinned structure is one of the most effective means of achieving the necessary levels of strength and toughness, while the thin films of retained austenite contribute the qualities of ductility and formability. Obtaining such a dislocated lath microstructure rather than the less desirable twinned structure is achieved by a careful selection of the alloy composition, which in turn affects the value of Ms.
The stability of the austenite in the dislocated lath microstructure is a factor in the ability of the alloy to retain its toughness, particularly when the alloy is exposed to harsh mechanical and environmental conditions. In certain conditions, austenite is unstable at temperatures above about 300xc2x0 C., tending to transform to carbide precipitates which render the alloy relatively brittle and less capable of withstanding mechanical stresses. This instability is one of the issues addressed by the present invention.
It has now been discovered that carbon steel alloy grains having the dislocated lath microstructure described above tend to form multiple regions within a single grain structure that differ in the orientation of the austenite films. During the transformation strain that accompanies the formation of the dislocated lath structure, different regions of the austenite crystal structure undergo shear on different planes of the face-centered cubic (fcc) arrangement that is characteristic of austenite. While not intending to be bound by this explanation, the inventors herein believe that this causes the martensite phase to form by shear in various different directions throughout the grain, thereby forming regions in which the austenite films are at a common angle within each region but at a different angle between adjacent regions. Due to the austenite crystal structure, the result can be up to four regions, each with a different angle. This confluence of regions produces crystal structures in which the austenite films are of limited stability. Note that the grains themselves are encased in austenite shells at their grain boundaries, while the inter-grain regions of different austenite film orientations are not encased in austenite.
It has further been discovered that martensite-austenite grains of a dislocated lath structure with austenite films in a single orientation can be achieved by limiting the grain size to ten microns or less, and that carbon steel alloys with grains of this description have greater stability upon exposure to high temperatures and mechanical strain. This invention therefore resides in carbon steel alloys containing grains of dislocated lath microstructures, each grain having a single orientation of the austenite films, i.e., each grain being a single variant of the dislocated lath microstructure.
The invention further resides in a method of preparing such microstructures by heat soaking (austenitization of) the alloy composition to a temperature that places the iron entirely in the austenite phase and all alloying elements in solution, then deforming the austenite phase while maintaining this phase at a temperature just above its austenite recrystallization temperature to form small grains of 10 microns or less in diameter. This is followed by cooling the austenite phase rapidly to the martensite start temperature and through the martensite transition region to convert portions of the austenite to the martensite phase in the dislocated lath arrangement. This last cooling is preferably performed at a rate fast enough to avoid the formation of bainite and pearlite and the formation of any precipitates along the boundaries between the phases. The resulting microstructure consists of individual grains bounded by shells of austenite, each grain having the single-variant dislocated lath orientation rather than the multiple-variant orientation that limits the stability of the austenite. The alloy compositions suitable for use in this invention are those that allow the dislocated lath structure to form in this type of processing. These compositions have alloying elements and levels selected to achieve a martensite start temperature Ms of at least about 300xc2x0 C., and preferably at least about 350xc2x0 C.